Thermoplastic tubing is used in a variety of applications, such as appliance input and output lines, garden hoses, automobile hoses, medical devices, etc. The tubing is often flexible and able to bend upon the application of an external force. Typically, upon the removal of the external pressure the tubing relaxes to its original position. Attempts have been made to create a tubing product that will retain a predefined shape, see for example U.S. Pat. No. 6,455,117 and Canadian Patent 1,229,313. However, to date no technology has been successful in creating a method to produce flexible tubing material that retains a nonlinear shape without impeding the flow of fluids and gases passing therethrough.
Plastics extrusion processing is defined as converting plastic powder or granules into a continuous uniform melt and forcing this melt through a die which yields a desired shape. This melted material must then be cooled back to its solid state as it is held in the desired shape, so an end product can be realized.
Single screw extruders are the most common in use today. Extruder diameters range from ½″ to 12″ in a barrel inner diameter. The hopper of an extruder accepts granules or powder which pass through a vertical opening in the feed section where they are introduced to a rotating screw with spiral flights. The material is conveyed along the screw and heated inside the barrel, with the goal being to reach the die system in a totally melt phase at an acceptable and homogeneous temperature, and being pumped at a consistent output rate.
The barrel is heated and cooled by heater/cooler jackets surrounding its outer wall to aid in the melting of the material on the screw. Heater/coolers are electrically heated through heating elements cast into aluminum, with either cooling tubes also cast into the aluminum or deep fins cast on the outer surfaces of the heaters/coolers to allow air cooling of the barrel via blowers. Temperature of the various barrel zones are set according to the material, screw design, and processing goals. These barrel zone temperature settings vary widely, depending on the material used or the product being made while the control of the temperature at the deep barrel thermocouple position for a given situation is typically maintained within a close tolerance range to minimize variations of material exiting the die system. The screw is the heart of the extrusion process and designs for which have varied with time as understanding of the melting process of the plastic material moving along the screw has increased. Since some materials tend to trap air as they start to melt, or contain moisture or volatiles, that will create porosity in the final product, a vent is typically positioned at a point in the barrel to remove the porosity by allowing the escape of gases.
The melt must be shaped and cooled by product sizing and cooling equipment to its solid phase while forming a product that falls within given size tolerances. The dies to create the end products from a melt are varied depending on the shapes involved. Pipe and tubing are cooled through simple, open water troughs, or pulled through vacuum sizing tanks, where the melt is held in a sizing sleeve for a short time in a water filled vacuum chamber. Custom profiles come in various shapes and are commonly made of materials that have high melt viscosity, so they are easy to hold shape while they cool. These products can be cooled by forced air, water troughs, or water spray methods. The methods of getting the many shapes include various sizing fixtures to hold the extrudate as it is pulled through the system and cooled. The material can also be coextruded, i.e., made with more than one material. Coextrusion typically requires a dual-extrusion head and multiple extruders using a specialized die system to bring these layers together with a common sizing and shaping system. Rates of 100 feet per minute are routinely achieved.
To accurately maintain diameter and wall thickness of polymer tubes, a uniform flow rate of melt from the extruder must be guaranteed. All extruders, even those designed for producing extremely tight tolerances will exhibit some surging as a result of electrical drive control fluctuations, screw design, and the normal rheological variation in the polymer. Clearly, higher than commercially acceptable reject rates and waste levels will result if the process relies solely on extruder stability.
Injection molding of thermoplastics is a process by which a polymer is melted and injected into a mold cavity void. The mold used to create the final part is the inverse shape of the desired final product. Molds are typically made of hardened steel or aluminum. Once the melted plastic is injected into the mold, it cools to a shape that reflects the form of the cavity. The result is a finished part needing no other work before assembly into or use as a finished part.
The injection molding machine has two basic components: an injection unit to melt and transfer the plastic into the mold; and a clamp to hold the mold shut against injection pressures and for parts removal. The injection unit melts the plastic before it is injected into the mold. It then injects the melt with controlled pressure and rate into the mold. After the injection cycle, the clamp gently opens the mold halves so the part can be removed from the mold.
Important factors in the processing of plastic for the injection molding process include temperature, consistency, color dispersion and density of the melt. Conductive heat supplied by barrel temperature and mechanical heat generated by screw rotation both contribute to the processing of good quality melt. Often, most of the energy available for melting the plastic is supplied by screw rotation. Mixing happens between screw flights and the screw rotates, smearing the melted surface from the plastic pellet. This mixing/shearing action is repeated as the material moves along the screw until the plastic is completely melted.
When the polymer is a thermoplastic, injection molding uses a screw or a plunger to feed the polymer through a heated barrel to decrease its viscosity, followed by injection into a heated mold. Once the material fills the mold, it is held under pressure while chemical crosslinking occurs to make the polymer hard. The cured part is then ejected from the mold while at the elevated temperature and cannot be reformed or remelted.
When thermoplastics are heated in an injection press, they soften and as pressure is applied, flow from the nozzle of the press into an injection mold at the injection points. The mold has cavities that, when filled with the thermoplastic material, define the molded part. The material enters these cavities through passages cut into the mold, called runners. The mold also has passages in it to circulate a coolant, usually water, through strategic areas to chill the hot plastic. As it cools, the thermoplastic material hardens. When cooled enough, the mold opens and the part is removed.
Injection molding of thermoplastics is increasingly regarded as the preferred method for delivering high quality, value added commercial parts. This process allows for high volume production of complex tightly toleranced three-dimensional parts.
Insert molding is a type of injection molding process. Insert molding builds on the technology of injection molding by placing an insert piece into the cavity of the injection mold before the melted thermoplastic is injected. As the injected melted plastic cools, it typically bonds with the insert piece to create a single object.
In one embodiment the melted plastic can create molecular or mechanical bonds with the insert piece, depending on the material of each. The insert piece can be a thermoplastic or a metal. If the insert material is the same or very similar to the thermoplastic of the injected melted plastic a molecular bond will form between the two. Molecular bonds have strong physical strength, as well as strong leak resistance. If the insert material and injected plastic are substantially different, no molecular bond will occur, but instead a mechanical bond will form by the shrinking of the injected material as it cools or by bonding of the irregularities in the surface of the insert by the injected material.
Standard injection molding presses can be used for insert molding, but special molding machine designs that are better suited for insert molding also exist. Specialized insert molding presses are designed with added features to ease the loading of the insert pieces into the mold, and to hold the insert pieces in place during the injection and hardening of the melted polymer.
The design considerations for insert molding are generally the same as the considerations for other types of injection molding, such as the rate of flow of the melted polymer, and the pressure and temperature of the melted injected polymer. Additional concerns unique to insert molding usually relate to the bonding between the insert piece and injection molding material. Examples of additional concerns are the material of the insert piece, the pull and compression strength requirements, the leak test requirements, and the torque or axial force requirements of the bond between the insert piece and the overmolded second polymer.